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Radiography of 140 mm Thick Weld- Multiple Film Technique S.P. S.P. Srivastava, Sriv astava, S.P S. P. Pandarkar, K.B. Santhosh, G.P. G.P. Sahu and S.B. Jawale Centre for Design and Manufacture
Abstract Radiography of 140 mm thick weld joints of Low Alloy Carbon Steel Cruciform Test Specimen was a challenging task due to radiation hazard hazar d attributed to large exposure time, tim e, high energy and high strength radiation source. High thickness of steel also adds to internal scatter within the specimen, reducing t he quality of radiographs. Multiple Mult iple film technique, which uses more than one film of same or different speed in a single cassette, was developed for panoramic exposure of four weld joints using cobalt 60 source. The technique was able to improve the quality of the radiographs besides reducing the exposure time by one third. This paper presents the detail of multiple film technique t echnique and its role in reducing the effect of thickness thi ckness variation variatio n on the image quality.
Introduction Under the Component Integrity I ntegrity Test Test Programme,
achieved by machining the weld edge to compound
Low Alloy Carbon Steel Cruciform Test Specimen
bevel angles, thus making the groove narrower than
are manufactured at Centre for Design and
the conventional groove. Ultrasonic Ultra sonic testing (UT) was
Manufacture (CDM) for biaxial load testing as a part
not feasible because of number of notches adjacent
of studies pertaining to safety aspects of Indian
to the weld joint. Since radiographic testing (RT)
Pressurised Heavy Water Reactor. Specimens are
was the only suitable NDE method for the
made out of ASTM 508 Low Alloy Carbon Steel (20
volumetric examination of the weld, it was employed
MoNi55) in the shape of cruciform having unequal
to detect lack of fusion (LOF) in the side wall, lack
arms each of length 3300 mm and 1900 mm. Four
of penetration (LOP) at the root, porosity, crack and
slabs, each 135 mm thick x 300 mm wide, are
any other discontinuities discontinuit ies existing in the weld. RT of
welded to a symmetrical cross having two equal
thick specimen has inherent problems such as
arms of length 175 mm (Fig. 1). Manufacturing
requirement of high strength source, very high
process includes: Cutting of centre block and arms,
internal scatter, scatter, high radiation hazard due to longer l onger
Machining of centre block and arms for edge
exposure time, low sensitivity for the portion away
preparation, Machining Machin ing of notches and slots in the
from the film. Any attempt to align the source with
centre block, Welding of arms with the centre block,
weld edge in order to detect lack of side wall fusion
Radiographic testing of weld joints and Post weld
gives rise to large thickness variation demanding
heat treatment.
different exposure time for different shots. At CDM,
As per the standard practice, symmetrical double do uble V
multiple film technique techniq ue for panoramic exposure was
groove was adopted for butt weld joints between
developed to radiograph all the four weld joints
arm and centre block, considering very high thickness
simultaneously. On account of reduced exposure
of 135 mm of the base metal. Further, in order to
time and less number of shots, this technique offers
limit the distortion, heat input was minimized by
advantages of less radiation radiatio n hazard, reduced scatter,
reducing the volume of weld deposition. This was
high sensitivity and high productivity. productivity.
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Fig. 1: Cruciform Test Specimen Selection of NDT Method
zero degree angle of incidence for better detectability
Welds, with thickness less than 50 mm are examined
of LOF. In this case, due to presence of slots in the
by RT due to ease of interpretation and availability
scanning area UT was difficult, hence RT was
of image record. However, in case of RT of thick
considered suitable NDT method for volumetric
weld, radiation hazards are more due to high
examination of the weld.
exposure time and high strength of radiation source
Theoretical Considerations for RT of Thick
required for radiography. Therefore, UT with no
Weld
radiation hazard is considered a better choice for
In RT, direction of radiation beam with respect to
thickness 50 mm and above. Further, UT as
the orientation of the flaw plays an important role
compared to RT, gives better sensitivity for planar
for the flaw detectability. If the central beam is
defects such as lack of fusion and crack. In order to
aligned with the major dimension of the flaw, the
detect lack of side wall fusion, welds are subjected
probability of flaw detection is very high. In groove
to ultrasonic testing using angle beam technique.
weld, side wall and root are more vulnerable as far
Particularly for thick weld, obstacle free large
as defect is concerned. Radiographic shooting sketch
scanning area on both sides of the weld shall be
(RSS) for butt weld with double V groove is shown
available so that sound beam hits the sid e wall at
in the Fig. 2. For single V groove weld having
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Fast film records thicker portion whereas slow film gives the detail of thinner portion. In the second case, in order to reduce the exposure time, normally applicable for very thick job, two films of the same speed are exposed together which reduces the exposure time by one half. Combination of these films, when viewed separately will not meet density range of 1.0 – 3.5 individually and therefore will not reveal any detail. Such exposed films are Fig. 2: RSS of double V butt weld in plate having thickness 80 mm or more
viewed together i.e. by super imposing one on
thickness less than 10 mm, even source at S1,
combined density of two films shall meet the density
aligned with root position can detect LOP in the
range of 1.3-4.0. In the technique developed at
root and LOF in the side wall as well. However, as
CDM in total two pairs of film were used, as
the thickness increases, three different shots with
explained in the subsequent paragraph, to get the
source positions S1, S2a and S3a are required in
advantage of both improved latitude and reduced
order to detect LOP at root and LOF at each face.
exposure time.
Object away from the film produces large
Radiographic Testing of Weld Joints of
geometrical unsharpness and therefore image of the
Cruciform
weld groove which is away from the film will not be discernable, if thickness of the weld is more than 80 mm. Hence, beyond 80 mm thickness it is required to take shot from both sides of the weld, if accessible, to limit the unsharpness. Source positions S2a, S3a, S2b, and S3b are suitable for detecting LOF in walls GH, CD, EF and AB respectively. In case of cruciform, since the weld metal thickness including reinforcement was 140 mm similar shooting scheme was adopted for full volumetric examination of the weld.
another in front of the viewing illuminator. The
Weld groove for butt joint in 300x135 mm cross section consists of symmetrical double V with 2 mm root land. Each V groove, includes two bevel angles; first one 75° starting from root up to 20 mm height and second one 24° starting from 20 mm up to 47.5 mm height (Fig. 3). Qualified welding procedure specification (WPS) and qualified welders were used for welding. All weld edges were checked for presence of any lamination by liquid penetrant test (PT). Root was fused by GTAW, and remaining weld was deposited by SMAW process. Pre-heating,
Multiple Film Technique- Theoretical Aspects
with inter pass weld temperature control was done
In multiple film technique of radiographic testing,
to avoid the formation of martensite and to prevent
two or more films kept in close contact, are exposed
hydrogen induced cracks. Post weld heat treatment
together either to cover the varying thicknesses of the component or to reduce the exposure time. Films in combination may be either of same speed or different speed depending upon the requirement. In case of component of varying thicknesses, to improve the latitude i.e. range of thicknesses covered on the radiograph within acceptable density range of 1.0 to 3.5, combination of fast and slow fil m is exposed together and each film is viewed separately.
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Fig. 3: Weld groove configuration
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was adopted basically for stress relieving. Root pass
shots with source at an angle with respect to weld
welding and inter stage welding were subjected to
axis WW’. Considering the symetricity of all the
RT and PT to detect any gross defect whose
welds in each arm with respect to central axis XX’,
elimination after final pass requires lot of material
it was decided to use panoramic shot (PS) to
removal by grinding process.
radiograph all the four weld joints simultaneously
In order to detect lack of fusion in the side wall
in one shot, in order to reduce the total exposure
each weld joint in each arm requires at least five
time. In this set up, source is required to be
shots- one straight and four inclined shoots, two
positioned either at PS1 or PS2 on the centra l axis
from each face. Though thickness penetrated in
XX’ of the cruciform such that the source to film
straight shot is 140 mm, interpretation is carried
distance (SFD) remains same for all the four we ld
out only for half the thickness i.e. 70 mm of the
joints with beam parallel to the walls inclined at 37o
weld which is closure to the film. Straight shot (SS)
with the vertical.
in which source and film are kept at SS1 and F1
Use of Multiple Film Technique for RT of
respectively (Fig. 4), the total volume covered, is
Cruciform
ABCDEx300 mm3 (Fig. 3). In this shot, because of
Straight Shot: For straight shot with source at
its diverging nature, penetrating radiation is more
SS1 and film at F1 (Fig. 4), multiple film technique
or less parallel to faces AB and CD which are inclined
was used in which two films of same speed loaded
at 12° with the vertical. Therefore, these shots are
in the same cassette were exposed to half the
suitable for detecting any LOF in faces AB & CD
exposure time required for single film. Such exposed
and LOP at the root CO. Similarly by keeping source
films were viewed together i.e. by super imposing
at SS2 volume MNOPQ can be examined with high
one on another in front of the viewing illuminator.
degree of confidence for detecting LOP in faces MN
Since penetrameter was kept on full thickness,
and PQ.
minimum SFD obtained by Equation 1 (Table 1) for
Since faces BC (or CD) and NO (or OP) are inclined
T = 140 mm, ug = 1.7 mm and
at 37o with the vertical and job thickness is more
950 mm. During radiography, actual SFD was
than 80 mm, to increase the probability for detecting
increased to 1100mm to minimize the divergence
LOF, it is necessary to take another radiograph with
of the beam with respect to the face AB or DE.
source in offset position, such that radiation beam
Exposure time for D7 single film using 40 curie
is parallel to these walls. This requires two inclined
cobalt source was estimated to 6 hour.
= 7.5 mm was
Inclined and Panoramic Exposure: Thickness across the weld cross section, seen by the radiation beam is different, because of the inclination and therefore it becomes essential to use multiple film to accommodate variation of optical density within acceptable limit. Radiographic setup for the panoramic exposure is shown in the Fig. 5. With its entire arm in horizontal plane, cruciform was kept above the source to take advantage of the shielding during panoramic exposure. Ray diagram, using AutoCAD was prepared to know Fig. 4: RSS for panoramic shot for all four arms
the source position, source to film distance and thickness variation for a set up which will give
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parallel to the face BC within 6° and geometrical unsharpness was within the limit. For source at 600 mm below the top surface of the cruciform on vertical axis XX’, thickness, angle, and SFD with exposure time for each combination, are tabulated in Table 2. Exposure time were calculated using Equation 2, which is basically applicable to narrow beam geometry and is meant only as a guide for knowing approximate exposure time. Actual exposure time is established by trials rather than Fig. 5: Set up for panoramic exposure
formula as it depends on the accuracies of many variables such as source size and its strength , SFD,
adequate coverage of the weld with central beam
film processing time/condition, scatter radiation etc.
parallel to face BC. Values of geometrical
For very thick specimen actual exposure time is less
unsharpness were calculated using Equation 1 for
than the calculated one, due to internal scatter
three SFDs and thicknesses as shown in the Fig. 6.
contributed by the specimen itself. During trial shot,
From the results of several iterations, with var ying
using fast film Agfa D7, it was observed that even
SFD and thickness, it was observed that for source
with low exposure time, film density for the thickness
at 600 mm, gamma ray from a point source was
Fig. 6: Ray diagram for panoramic exposure using multiple film
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Fig. 7: Single film with 1.63 optical density Ta and SFDa, was on higher side and therefore to offset this, slow film Agfa D4 was considered a better option as compared to Agfa D7 film. To reduce the exposure time, combination of fast (D7) and slow (D4) films was found suitable for
Fig. 8: Double film with 3.32 optical density Interpretation of Multiple Films During interpretation of radiographs, two D7 films were viewed together for thicknesses ‘Tb’ and ‘Tc’. For thickness ‘Ta’, because of adequate density only one film either D7 or D4 can be viewed individually,
lower thickness region ‘Ta’ whereas combination of
however, if D7 and D4 are viewed together, better
two fast films (D7) were adequate for higher
perception of the IQI image is obtained. As per the
thickness regions ‘Tb’ and ‘Tc’. To take care of the
ASME requirements, image of the 4T hole in 60
continuous variation in the thickness and for the continuity of the weld image, one large size D7
number plate type IQI was seen when both the films were viewed together. In addition to plate type IQI,
film, overlapping the total thickness range was used.
wire type IQI No.1 ISO 7 (DIN) was also used, for
All the three films with their relative positions, were
which 5th wire image was seen. Weld at intermediate
loaded in a single cassette. Film were exposed for 3 hours 30 minutes, which is approximately one half the maximum exposure time T b = 6 hours 12 minutes and greater than the half of the minimum exposure time i.e. 3.9/2 hours.
stage having thickness of 110mm was radiographed using two films of same speed at half the exposure time required for single film. Fig. 7 and Fig. 8 are radiographic images of single film and superimposed double film respectively. On single film, with optical
Tungsten arrow markers were placed on the film as
density 1.63, even image of the fourth wire of a
well source side to show the coverage of the weld
wire type penetrameter is not seen clearly, whereas
width and projection of the source side weld image
two films superimposed and viewed together is able
on the film. In total three image quality indicators
to reveal well defined image of the fifth wire and
(IQI), two for source side and one for film side were
slightly faint indication of the sixth wire. Besides
placed to check the adequacy of the image quality.
wire, images of letter B and J4 are clearly seen in Fig. 8.
Table 3: Comparison between single and multiple film technique S. No. Exposure Type
Source Position
Shots per arm
Time per shot (hour)
Shots for four arms
Total Time (hour) Single Film Multiple Film
1.
Directional
SS1 & SS2
2
6
8
48
24
2.
Directional
PS1 & PS2
2
6.2
8
49.6
24.5
3.
Panoramicl
PS1 & PS2
2
6.2
2
12.4
6.2
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Conclusions
Multiple film technique with panoramic exposure
Radiography of thick weld requires proper planning
not only reduced the exposure time but also
to take care of flaw detectability, scatter radiation,
minimized the density variation along the composite
high exposure time, large value of unsharpness,
film from one end to another end.
shielding to reduce the radiation hazard, IQI
References
sensitivity etc. At CDM, multiple film technique was
1.
Richard A. Quinn and Claire C. Sigl,
developed to address these problems. Review of
Radiography in Modern Industry, Fourth edition.
exposure time (Table 3) indicates that total exposure
EASTMAN KODAK COMPANY, Rochester, New
time for four welds of a cruciform is 97 hour when
York 14650
single film is used with directional exposure. Whereas time for panoramic exposure combined with multiple film technique is only 30.2 hour, which is approximately one third (31%) of the total time.
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2. Nondestructive Inspection Methods Manual, Section VI, Special Radiographic Techniques, TM-1-1-1500-335-23, Integrated Publishing, http://www.tpub.com
